(362e) Novel Materials for Cold Energy Storage

Novel Materials for Cold Energy Storage

Ali A. AlNajjar,¹
H. Al Araj,¹ D. Bahamon,^1,2 , TJ Zhang,³ L.F.
Vega,^1,2,4 M. Khaleel^1,2

¹Chemical Engineering Department, Khalifa
University of Science and Technology, Abu Dhabi, UAE

²Center for Catalysis and Separation (CeCaS)
and Research and Innovation Center on CO₂ and H₂ (RICH),
Khalifa University of Science and Technology, Abu Dhabi, UAE

³Mechanical and Materials Engineering
Department, Khalifa University of Science and Technology, Abu Dhabi, UAE.

⁴Gas Research Center, Khalifa University of
Science and Technology, Abu Dhabi, UAE

Email: maryam.khaleel@ku.ac.ae

The
demand for energy worldwide is increasing due to the rapid development of
societies and this has led to more awareness of the problems of pollution and
energy recovery. For instance, the combined waste heat of China’s industries
corresponds to the combustion energy of 340 million tons of standard coal each
year [1]. In addition, more than 80% of energy comes from
nonrenewable resources that are fossil-fuel based while only less than 20% of
energy comes from renewable resources [2].
This imposes negative carbon footprint and since these resources are
nonrenewable, they are nonreliable for the future as they will get depleted
with time. Therefore, to overcome this energy crises it is important to use
renewable resources or enhance the efficiency of energy-intensive processes by
utilizing waste heat generated from these processes. According to UAE State of
Energy Report in 2015 [3],
UAE is considered amongst the highest electricity consumers in the World as the
demand for electricity in 2013 was 105 billion KWh and it is expected to
increase by 9% annually. In order to deal with the high energy demand, the UAE
plan to generate 27% of power through clean energy by 2021.

An
example of an energy intensive process that is widely used especially in hot
and arid countries is cooling. Cooling processes consume a significant
percentage of the total produced electricity in hot and arid countries. According to International Energy Agency (IEA) [4], air conditioning
consumes around 10% of the World’s total energy and air conditioning units are
expected to increase from 1.6 billion today (2018) to 5.6 billion by 2050,
which will triple the global energy required for air conditioning. Currently used cooling
processes are based on vapor compression refrigeration cycle (VCC).

An alternative to VCC is adsorptive heat transformation.
This involves the adsorption and desorption of guest molecules onto the surface
of a solid. The evaporation of a working fluid produces desired cold in the
cooling application while adsorption of the fluid vapors into the adsorbent
releases heat into the environment. Waste heat can be used to regenerate the
adsorbent and complete the cycle, which is equivalent to the compression in
conventional VCC. Energy storage will act as modulator between the supply and
demand of energy and will help in shifting the cooling load from daytime to
night time. Compared to other forms of energy storage (e.g. latent heat,
sensible heat and thermochemical reactions), thermal energy storage by
adsorption is desired since it is able to store energy indefinitely through a
chemical potential, can be operated in continuous cycles and can be driven by waste
heat sources [2].
One of the major challenges in adsorption thermal energy storage applications
is choosing the optimal working pairs (i.e. fluid and material) to obtain the
most desirable and efficient adsorption system. Therefore, an adsorbent with
certain characteristics should be chosen to reach the optimum goal. A great
adsorbent should have many characteristics, which include high energy density,
high energy efficiency, high adsorbent capacity, high stability of material and
system, high affinity between adsorbent and adsorbate, low temperature of
desorption and high heat of adsorption [2].

Previous
studies focused on porous materials such as zeolites, activated carbon and
silica gel. However, these materials are difficult to be tuned and
functionalized [5].
On the other hand, newer studies focus on exploring metal-organic frameworks
(MOFs) due to their characteristics such as tunability, high surface area,
large pore volumes, thermal and chemical stability, that makes them great candidates
for thermal energy storage [5–8].

The
aim of this work is to further explore MOF materials since they proved their
ability in thermal energy storage applications seeking for the best-performing
material. Different MOF materials are being synthesized in the lab and then
characterized using various characterization techniques such as X-ray
Diffraction (XRD) and Scanning Electron Microscopy (SEM). The synthesized
materials are then tested for adsorption using eco-friendly refrigerants that
have low Global Warming Potential. Factors studied include the effect of
structure and active sites on working capacity and stability. Results are
compared with molecular simulations to provide better understanding of the effect
of the interaction of the refrigerant with the MOF framework on the final
performance of the material and hence, as a tool to help in the optimization of
the pair refrigerant-adsorbent for this selected application.

We acknowledge support for
this work from Khalifa University of Science and Technology (project CIRA 121)

References:

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